The terms “magma” and “lava” are often used interchangeably, leading to misunderstandings about Earth’s dynamic processes. While both refer to molten rock, their distinct definitions are based on their location. This article clarifies the differences between magma and lava, detailing how their environment influences their properties and the geological formations they create.
The Fundamental Distinction
Magma is molten or semi-molten rock found beneath the Earth’s surface, residing within the crust or upper mantle. This superheated material, typically ranging from 700° to 1,300° Celsius (1,292° to 2,372° Fahrenheit), contains dissolved gases, suspended crystals, and sometimes fragments of solid rock. Magma forms when extreme temperatures and pressures deep within the Earth cause solid rock to melt, often in areas like subduction zones, mid-ocean ridges, or hotspots. It collects in large underground reservoirs known as magma chambers, which can be located several kilometers below the surface.
In contrast, lava is molten rock expelled from the Earth’s interior onto its surface. This expulsion typically occurs through volcanic eruptions or fissures. The primary difference between magma and lava lies solely in their location. Once magma emerges from a volcano or a crack in the Earth, it is termed lava. Despite this change in name, the fundamental chemical composition of the molten rock remains largely similar, although the loss of dissolved gases affects its properties.
Characteristics and Behavior
The location of molten rock influences its characteristics and how it behaves. Magma, contained beneath the Earth’s surface, holds dissolved gases such as water vapor and carbon dioxide under immense pressure. As this magma rises and nears the surface, the confining pressure decreases, allowing these dissolved gases to expand and form bubbles. This exsolution of gases can affect the magma’s behavior, often leading to more explosive eruptions when it becomes lava.
Lava, once it reaches the surface, experiences rapid cooling due to exposure to cooler air or water. While magma temperatures can range up to 1,300°C, freshly erupted lava usually ranges from 700° to 1,200°C. This quicker cooling rate causes lava to solidify faster than magma, which cools slowly underground due to the insulating effect of surrounding rock. The escape of gases and the cooling process also impact the molten rock’s viscosity, or resistance to flow. Some lavas can be very fluid, flowing like syrup, while others are stiff and move slowly.
What Happens After Eruption
Once lava erupts onto the Earth’s surface, it begins the process of solidification, transforming into solid rock. This rapid cooling and hardening of lava forms extrusive igneous rocks, also known as volcanic rocks. Examples of these rocks include basalt, common on the ocean floor, and obsidian, a volcanic glass formed when lava cools almost instantly. The rate at which lava cools directly influences the texture of the resulting rock.
When lava cools quickly on the surface, the mineral crystals within it have little time to grow large, resulting in fine-grained or glassy textures. In contrast, magma that remains trapped deep underground cools much more slowly, sometimes over thousands or millions of years. This extended cooling period allows mineral grains to grow to a larger, more visible size, forming coarse-grained intrusive igneous rocks, also known as plutonic rocks, like granite.